Conventionally, inorganic photoconductive materials such as selenium, cadmium sulfide, or zinc oxide have been used widely for the photosensitive layer of the electrophotographic photoreceptor. However, these materials are disadvantageous in that: selenium and cadmium sulfide need to be recovered as toxic substances; that selenium has poor heat resistance because it crystallizes by heating; that cadmium sulfide and zinc oxide have poor moisture resistance; and that zinc oxide does not have printing durability. Thus, an attempt has been made continuously to develop new photoreceptors. Recently, researches on application of organic photoconductive materials to the photosensitive layer of the electrophotographic photoreceptor have been progressed and some of them have been put into practical use. Compared with inorganic photoconductive materials, organic photoconductive materials are advantageous in that: their weights are light; that they can be easily formed into film; that photoreceptors containing them can be easily produced; that some of them enable productions of transparent photoreceptors; and that raw materials are harmless.
Recently, development of the so-called function distribution type of the photoreceptors in which functions of generation and transfer of a charge carrier are distributed to different compounds, is recent main current, because this type is effective for high sensitivity. An attempt has been made to practically use this type of organic photoreceptors.
As a charge carrier-transferring medium, either a high molecular photoconductive materials such as polyvinyl carbazole or a low molecular photoconductive compound which is dissolved and dispersed in binder polymer, is used.
Particularly, photoreceptors excellent in mechanical properties can be easily produced by using organic low molecular photoconductive compounds, since it is possible to select polymer having excellent film-forming property, flexibility, and adhesive property (see, for example, Japanese Patent Laid-Open Publication No. 63-269160, Japanese Patent Publication Nos. 3-39306 and 4-53308, etc.). However, it is difficult to find materials suitable for producing highly sensitive photoreceptors.